The present invention relates to a process for the introduction of a catalyst into a gas-phase olefin polymerization reactor, in particular into a fluidized bed reactor.
The catalysts used in gas-phase olefin polymerizations are often provided in the solid state. The solid catalysts can be kept either in the form of a suspension in a liquid hydrocarbon or in the form of a dry powder. Now, it has been observed that some solid catalysts which are particularly active in polymerization have properties which deteriorate when they are kept in suspension. There therefore appears a need to keep solid catalysts in the form of a dry powder. Moreover, many catalysts are now manufactured and delivered in this form because they can be transported and handled more easily.
It is known that solid catalysts used in the form of a dry powder are generally introduced in this form into gas-phase olefin polymerization reactors using in particular a gaseous fluid such as a carrier gas, for example nitrogen, hydrogen, a gaseous olefin or a mixture of these gases. This method of introduction, carried out entirely in the absence of liquid, is, for example, described in French Patent Application FR-2,705,252-A. However, it has been observed that this method of introduction can cause one or a number of problems characterized in particular by the use of large volumes of gaseous fluids introduced with the catalyst into the reactor, an insufficiently homogeneous dispersion of the catalyst in the reactor, an excessive entrainment of the catalyst out of the fluidized bed, and the appearance of hot spots, both in the fluidized bed and above the bed. These problems can arise separately or simultaneously, depending on the type of catalyst used, in particular depending on the composition or the size of the catalyst.
Patent Application of Luxembourg LU-A-79915 discloses a process for introducing a solid catalyst into a liquid phase polymerization reactor. The catalyst in the form of a dry powder is contacted with a liquid in two successive settling zones wherein the catalyst falls by gravity under the protection of the liquid. Contacting the catalyst in the form of a dry powder with the liquid neither produces any mixing of entrained catalyst with the said liquid, nor forms a suspension of the catalyst in the said liquid. In such a contacting step, the liquid is used as a screen for protecting the catalyst, and not as a carrier for suspending and transporting the catalyst into the polymerization reactor. Once the catalyst falls by gravity through the settling zones under the protection of the liquid, it is then entrained into the polymerization reactor with the help of a liquid stream. The process thus involves relatively large amounts of liquid which may affect the catalyst activity.
A process has now been found which makes it possible to very substantially reduce or even to completely avoid the above-mentioned problems. The process has the advantage of being able to make use of catalysts which are different both in their composition and in their size and providing a more universal process which makes it possible to use different catalysts in the same reactor.